The invention relates to a method and associated circuit for detecting capacitances and capacitance changes at sensor electrodes.
Capacitive proximity switches are known in the prior art in the field of application for vehicles. By way of example, EP 1505734 discloses a capacitive proximity switch and an associated evaluation process. Capacitive proximity switches of this type have sensor electrodes and should respond when a certain capacitance or capacitance change is detected between the sensor electrode and a reference electrode. For this, numerous electrodes are frequently disposed in the region of a vehicle configured for detecting operation.
For detection, the sensor electrode is brought to a predefined potential, and forms a capacitance with a reference electrode, e.g. the vehicle ground or the ground of the surface located beneath the vehicle. This capacitance can change when a body moves in the sensitive region of the sensor assembly, e.g. the hand of a user or the foot of a user.
There are different fundamental explanations for this effect. On one hand, the dielectric surrounding the sensor electrode changes, and on the other hand, the body of the user forms a (conductive) electrode, such that its proximity corresponds to a coupling having another capacitance. In any case, the change in the environment of the sensor electrode caused by the operator body part leads to a change in the electrical field and to a charge transfer, and thus to the measurable capacitance. Independently of the explanation, the capacitance of the sensor electrode having a short-term variability, however, can be detected and indicates changes in the environment.
The detection of the capacitance itself can be achieved in a variety of ways. By way of example, a number of clocked discharge procedures or a discharge period can be used as the measurement value for the capacitance. Evaluation methods of this type are sufficiently known in the prior art. By way of example, a capacitive detection of a proximity to a vehicle door handle is known from DE 19617038A1.
One problem with the known sensor assemblies is that external factors lead to the capacitance measured at the sensor electrode being able to change without the approach of a user. The capacitance of a sensor electrode is highly dependent on the environment, e.g. the air humidity, a coating with rain, snow, ice or dirt in the region of the sensor electrode. In this context, a capacitive proximity sensor is known, for example, from DE 19620059A1, which only switches when the capacitance between the response electrode and the reference electrode changes at a rate that exceeds a lower threshold value.
These concepts do not change the fact that the evaluation circuit is normally configured to an operating point that corresponds to an expected capacitance or change in capacitance that is to be evaluated. If the capacitance is significantly changed by external factors, then the precision of the evaluation also sinks, because the configuration of the circuit no longer corresponds to the base value of the capacitance.
Moreover, it is always desirable to try to optimize the evaluation circuit with respect to the detection rate and reduction of the number of, or costs for, the components that are used, or the construction costs.
The object of the invention is to provide an improved evaluation circuit for detecting the capacitance at sensor electrodes.